Organic light emitting devices (OLEDs), which make use of thin film materials that emit light when excited by electric current, are becoming an increasingly popular form of flat panel display technology. There are presently three predominant types of OLED construction: the "double heterostructure" (DH) OLED, the "single heterostructure" (SH) OLED, and the single layer polymer OLED. In the DH OLED, as shown in FIG. 1A, a transparent substrate 10 is coated by an anode layer 11. A thin (100-500 .ANG.) organic hole transporting layer (HTL) 12 is deposited on the anode 11. Deposited on the surface of the HTL 12 is a thin (typically, 50 .ANG.-500 .ANG.) emission layer (EL) 13. The EL 13 provides the recombination site for electrons injected from a 100-500 .ANG. thick electron transporting layer 14 (ETL) with holes from the HTL 12. Examples of prior art ETL, EL and HTL materials are disclosed in U.S. Pat. No. 5,294,870, the disclosure of which is incorporated herein by reference.
The device shown in FIG. 1A is completed by the deposition of metal contacts 15, 16 and a top electrode 17. Contacts 15 and 16 are typically fabricated from indium or Ti/Pt/Au. The electrode 17 is often a dual layer structure consisting of an alloy such as Mg/Ag 17' directly contacting the organic ETL 14, and an opaque, high work function metal layer 17" such as gold (Au) or silver (Ag) on the Mg/Ag. When proper bias voltage is applied between the top electrode 17 and the contacts 15 and 16, light emission occurs from the emission layer 13 through the substrate 10.
The SH OLED, as shown in FIG. 1B, makes use of multifunctional layer 13' to serve as both EL and ETL. One limitation of the device of FIG. 1B is that the multifunctional layer 13' must have good electron transport capability. Otherwise, separate EL and ETL layers should be included as shown for the device of FIG. 1A.
A single layer polymer OLED is shown in FIG. 1C. As shown, this device includes a glass substrate 1 coated by an anode layer 3. A thin organic layer 5 of spin-coated polymer, for example, is formed over the anode layer 3, and provides all of the functions of the HTL, ETL, and EL layers of the previously described devices. A metal electrode layer 6 is formed over organic layer 5. The metal is typically Mg or other conventionally-used low work function metal.
Light emission from OLEDs is typically via fluorescence or phosphorescence. Successful utilization of phosphorescence holds enormous promise for organic electroluminescent devices. For example, an advantage of phosphorescence is that all excitons (formed by the recombination of holes and electrons in an EL), which are triplet-based in phosphorescent devices, may participate in energy transfer and luminescence in certain electroluminescent materials. In contrast, only a small percentage of excitons in fluorescent devices, which are singlet-based, result in fluorescent luminescence.
In comparison to fluorescent devices, however, phosphorescent devices have several potential drawbacks that must be overcome to produce useful electroluminescence. For example, energy transfer is relatively slow, as long range dipole-dipole coupling (Forster transfer) is forbidden by spin conservation. Efficiency also decreases rapidly with current density, long phosphorescent lifetimes which cause saturation of emissive sites, and triplet-triplet annihilation. In addition, triplet diffusion lengths are typically long (e.g., &gt;1400 .ANG.) compared with typical singlet diffusion lengths of about 200 .ANG.. Thus, if phosphorescent devices are to achieve their potential, device structures need to be optimized for triplet properties.